A computer, which supports modern civilization, operates by means of an electrical current, which is a flow of electrons. An electronic device in which this electrical current is operated and applied to recording and/or erasing information is constituted by a semiconductor. The electrons that flow in the semiconductor are scattered by impurities and Coulomb force, and thus generate Joule heat.
For this reason, a computer requires a fan for cooling. Further, a part of an input energy cannot be utilized for recording and erasing information due to the Joule heat mentioned above, and energy loss thus occurs. Namely, there can be no doubt that suppression of the scattering of electrons is a major technical development achieved for power saving of the electronic device mentioned above.
As one solution, there has been a method of suppressing the scattering mentioned above of electrons conventionally by operating the electronic device mentioned above at an extremely low temperature. For example, use of a superconductor corresponds to such a method. Since scattering of electrons becomes zero in the superconductor mentioned above, there is no electrical resistance, and no Joule heat is generated. Therefore, the above-mentioned scattering of electrons does not occur.
However, in a case where this method is used, it is necessary to cool the electronic device to a temperature of several Kelvins, and an energy consumed for this purpose cannot be neglected. Further, it is difficult to generalize an electronic device utilizing such an extremely low temperature state and put this electronic device into practical use. Therefore, under the current situation, there is no satisfactory one as means capable of suppressing the scattering of electrons at a room temperature.
However, the situation has been changing since around 2007. This is because a theoretical prediction of a topological insulator was suggested as a physical theory. The topological insulator is an insulator in which a special electronic state that occurs only on a surface or an interface of an object is utilized, and is explained based on a relativistic effect that occurs due to the relativistic velocity close to light of inner-shell electrons of a relatively heavy chemical element.
Namely, by this effect of electrons (a spin-orbit interaction), an energy term of a spin-orbit interaction is added to the Hamiltonian of a band structure formed by the electrons, and a change in band structures and energy-inherent values occurs. At this time, in a certain substance, the uppermost band of a valence band at a vacuum surface is connected to the lowermost band of a conduction band, whereas a band gap opens in the bulk state.
As a result, a special physical property that has not been known until now appears. The physical property is such that the substance becomes a conductor on a surface or at interfaces thereof, whereas the bulk becomes an insulator. A substance having such a property is referred to as “topological insulator” (see H. Zhang et al. Nature Physics, 5, 438 (2009)).
A special electron band structure possessed by the topological insulator mentioned above has a non-trivial characteristic that the electrons present on the surface or at interfaces of the substance are divided into two electron spin flows each having a different spin due to time reversal symmetry and the electrons continuously flow without application of any electric field. In other words, this is the same as a fact that the topological insulator has an important property in which the topological insulator does not undergo the scattering mentioned above of electrons due to the impurities and the like. Further, for example, in a case where there is no external magnetic field that breaks the time reversal symmetry mentioned above, this property is kept very strongly. The name of the above-mentioned topological insulator is derived from a fact that such a property possessed by the electron band structure has a similar property to that of the topology phase theory of mathematics (see H. Zhang et al. Nature Physics, 5, 438 (2009)).
After the presence of the topological insulator mentioned above has been predicted in theory, a search for materials that actually have this non-trivial property was started. As a result, bismuth-tellurium alloys, antimony-tellurium alloys, and the like, which have high crystallinity, were confirmed by experiments by means of photoelectron spectroscopy. However, the single crystals used in these experiments were manufactured by a method of cooling a molten alloy or the like, and cannot be directly applied to the electronic device mentioned above (see Y. Xia et al. Nature Physics, 5, 398 (2009)).
On the other hand, the present inventors suggested a superlattice-type phase change solid-state memory without any relation to the topological insulator mentioned above in order to decrease an electrical power consumed by a phase change type solid-state memory. The superlattice-type phase change solid-state memory has a superlattice-type phase change film formed by laminating crystal alloy layers formed of antimony-tellurium and crystal alloy layers formed of germanium-tellurium so as to share the (111) plane axes and the c axes possessed by the respective crystal alloy layers. In the superlattice-type phase change solid-state memory, memory operations are possible by switching an array structure of the germanium atoms in an axis direction of crystal growth (see Japanese Patent No. 4,621,897 and Japanese Patent No. 4,635,236, and Tominaga et al. Nature Nanotechnology, 6, 501 (2011) and J. Tominaga et al. Applied Physics Letter, 99, 152105 (2011)).
Further, the present inventors provided a spin memory that can accumulate spin flows generated by injecting electrons thereinto while applying an electric field in a vertical direction, by utilizing that a superlattice-type phase change solid-state memory can be an ideal topological insulator (see International Patent Publication No. 2013/125101).
In addition, the present inventors suggested a transistor that is configured to apply a voltage by using a superlattice structure possessed by a superlattice-type phase change solid-state memory as a gate to control an electrical current (spin flow) that flows in the plane of the superlattice structure (see Japanese Patent Application Publication No. 2015-35478).
A superlattice structure, in which crystal alloy layers each having a ratio of antimony atoms to tellurium atoms of 2:3 and having aligned crystalline orientation (hereinafter, referred to also as “Sb2Te3 layers”) and crystal alloy layers formed of germanium atoms and tellurium atoms and having aligned crystalline orientation (hereinafter, referred to also as “GeTe layers”) are repeatedly laminated, is constituted by two different crystal forms (phases) called as a set phase and a reset phase (see J. Tominaga et al. Sci. Technol. Adv. Mater. 16, 014402 (2015)).
Since the reset phase mentioned above has two properties including space reversal symmetry and time reversal symmetry, each spin band degenerates and does not have magnetism.
The set phase mentioned above loses the space reversal symmetry because one of a pair of germanium atoms inverts to a side of a Sb2Te3 layer. However, since the time reversal symmetry is maintained, a spin split band called as a Rashba effect is formed.
In this split band, a formula “E (k, down spin)=E (−k, up spin)” is established from a conservation law of the time reversal symmetry at a band space made by energy (E)—momentum (k), and thus the split band cannot freely have a spin state. Namely, the scattering of electrons is significantly limited.
Further, since the band is spin-split in a state of the set phase in the GeTe layer mentioned above, the band is magnetized when an external magnetic field is applied thereto. Moreover, a magnetic moment is generated when an electric field is applied thereto. Conversely, an electric field is generated when a magnetic field is applied thereto.
Namely, the superlattice structure mentioned above simultaneously has an electric dipole and a magnetic moment such that a magnetic moment is generated when an electric field is applied thereto and an electric field is generated when a magnetic field is applied thereto. A property of simultaneously having both an electric dipole and a magnetic moment is called as multiferroic.
As materials that exert the multiferroic, one that exerts at an extremely low temperature is known (see Takahisa Arima, Multiferroics New development of electromagnetics in material (2014)). Since the superlattice structure mentioned above exerts the multiferroic under a temperature condition at a room temperature or higher, it can say that the superlattice structure is a highly practical multiferroic material.